Trench capacitors are in use in electronic circuits as decoupling capacitors, also referred to as bypass capacitors herein, in a circuit configuration, wherein the capacitor is inserted in parallel between a signal line and ground potential.
In this configuration, trench capacitors offer the advantage of a small impedance, which is given byZ=1/jωC  (1)
Here, Z denotes the impedance, ω is the circular frequency, and is related to a frequency f of a signal by ω=2πf, C is the capacitance, and j is the well known imaginary unit number. Trench capacitors typically have large capacitance values, and therefore represent a nearly perfect short circuit between the signal line and ground potential for alternating current (AC) signals. Equation (1) implies that the higher the capacitance, the better the short circuit to ground.
FIG. 1 shows a known trench capacitor structure 100 for achieving high capacitance values. This trench capacitor structure was published in F. Roozeboom et. al., “High-Density, Low-Loss MOS Capacitors for Integrated RF Decoupling”, Int. J. Microcircuits and Electronic Packaging, 24 (3) (2001) pp. 182-196. The trench capacitor 100 of FIG. 1 is embedded in a silicon substrate 102 and comprises an array of coupled layer structures, of which layer structures 104 to 112 are shown in a cross-sectional view. The layer structures consist of an array of U-shaped cross-sectional profiles in a plane perpendicular to a top side 114 and a bottom side 116 of substrate 102. The U-shaped layer structures are identical, and corresponding layers of the layer structures are connected with each other. The layer structure comprises a first, lower electrode 118, which is formed by an n+-silicon layer. This n+-layer is present on an n−-doped silicon substrate. A dielectric layer 120, which can for instance be a oxide/nitride/oxide layer stack of 30 nm thickness, isolates the lower electrode 118 from an upper electrode 122, which is made of an n+ polysilicon layer. A metal top electrode 124 is deposited on top of the upper electrode 122.
The U-shaped layer structure 118 to 122 may typically be formed in a pore having a diameter of 2 μm and a depth 102 of 20 to 30 μm. Typical capacitance densities per area are between 25 nF/mm2 to 75 nFmm2 are reached with a capacitor device according to FIG. 1.
FIG. 2 schematically shows a circuit configuration 200 with a prior-art trench capacitor device 202 used as a bypass capacitor between a signal line and ground potential. The trench capacitor structure 100 of FIG. 1 is suitable for use in the capacitor device 202 due to its large capacitance. However, for use as a bypass capacitor in the present circuit configuration, a ground contact must be provided on the bottom side 116 of substrate 102, which is labeled by reference number 206 in FIG. 2. The ground contact 206 is connected to ground potential. Capacitor device 202 further has a contact structure 204 with two ports 204.1 and 204.2 for signal input and output, which are connectable to a signal line (not shown). Theoretically, if the capacitor device 200 could provide a perfect short circuit to ground, a signal wave entering at port 1 204.1 would fully be reflected. Therefore, port 2 204.2 would perfectly be decoupled from port 1 204.1.
However, the performance of known trench capacitors is dependent on frequency and far from providing a perfect short circuit to ground in the circuit configuration of FIG. 2. This will be explained in the following with reference FIG. 3. FIG. 3 is a diagram showing the dependence of the S21 transmission coefficient of a prior-art trench capacitor as a function of frequency. The frequency is plotted in Hertz (Hz) on a logarithmic scale. The transmission coefficient S21 is given in units of dB. Three measured curves are shown for three different trench capacitors having capacitances of 2.2 nF, 22 nF, and 380 nF, respectively. All three curves show a decrease of the transmission coefficient S21 in a frequency range between 1 MHz and about 50 MHz (labeled “Range I”). A resonance effect in the shown transmission characteristics is seen for each curve, occurring between 100 MHz and 1 GHz, depending on the capacitance value.
The shown frequency dependence is due to the self-inductance of the trench capacitor. At the self-resonance frequency, the capacitance C and the self-inductance Lself of the trench capacitor are in resonance. Here the operation of the capacitor is best, e.g. maximum signal suppression occurs, although the suppression in the range of GHz is still much better than with discrete SMD placed capacitors.
Table 1 below shows a comparison of surface area, capacitance, C, resistance, R, and self-inductance, Lself, values for several prior-art trench capacitors.
TABLE 1Comparison of surface area, capacitance, resistance, and self-inductancevalues for several prior-art trench capacitors in an n−-substrateCapacitor Surface [mm2]C [nF]R [mΩ]Lself [pH]0.1022.2159560.3848.580251.042248223.5080.526119.1221316819.138094
Industrially, the contact to ground is however present at the front side. In order to provide a network of capacitors and inductors, it is necessary to use a high-ohmic substrate, with a substrate resistivity in the order of 1 kΩcm or more. A current-path through the substrate to the bottom side 116 would have a far larger resistance, and thus a less adequate self-inductance.
However, there is currently a trend towards broadband applications. This is for instance the result of the UMTS protocol, which has a broader bandwidth than GSM. It is also encouraged by the IEEE 802.16 protocol. Additionally, the number of bands increases, particularly above 2 GHz, i.e. for Bluetooth, W-LAN and other wireless standards. For such broadband applications, there is a need that the self-inductance is low. Evidently, the broadband generally includes a portion above the resonance frequency (“Range II”). Here, the signal suppression between port 1 204.1 and port 2 204.2 is less efficient and a significant amount of the incident wave is transmitted to port 2. The higher the self-inductance, the less efficient the signal suppression. Also, the trench capacitor with the contact to ground at the front side is not adequate, as the path to ground tends to be long.